The present invention relates to drying fibrous structures, and particularly to drying fibrous structures using a limiting orifice drying media.
Fibrous structures have become a staple of everyday life. Although the method of the present invention is particularly useful for drying wet laid fibrous structures as herein disclosed, the method is not considered limited to to such an application. The method may also be used to dry nonwoven structures of synthetic, natural or a combination of fibers. The method can also be used for drying woven fibrous structures as well.
Cellulosic fibrous structures are found in facial tissues, toilet tissues and paper toweling. One advance in the art of cellulosic fibrous structures is to provide multiple regions in the cellulosic fibrous structure. A cellulosic fibrous structure is considered to have multiple regions when one region of the cellulosic fibrous structure differs from adjacent regions of the cellulosic fibrous structure by at least one intensive property including but not limited to: basis weight, density, opacity, permeability, and predicted average pore size.
In the manufacture of cellulosic fibrous structures, a slurry of cellulosic fibers dispersed in a liquid carrier is deposited onto a forming wire creating a wet web. Any one of, or combination of, several known means may be used to dry the wet web. Each drying means will affect the properties of the resulting cellulosic fibrous structure. For example, the drying means and process can influence the softness, caliper, tensile strength, and absorbency of the resulting cellulosic fibrous structure. Also the means and process used to dry the cellulosic fibrous structure affect the rate at which it can be manufactured, without being rate-limited by such drying means and process.
An example of one drying means is felt belts. Felt drying belts have long been used to dewater a cellulosic fibrous structure through capillary flow of the liquid carrier into a permeable felt medium held in contact with the web. Dewatering a cellulosic fibrous structure into and by using a felt belt results in overall uniform compression and compaction of the cellulosic fibrous structure web to be dried.
Felt belt drying may be assisted by a vacuum, or may be assisted by opposed press rolls. The press rolls maximize the mechanical compression of the felt against the cellulosic fibrous structure. Examples of felt belt drying are illustrated in U.S. Pat. Nos. 4,329,201 issued May 11, 1982 to Bolton and 4,888,096 issued Dec. 19, 1989 to Cowan et al. One issue associated with the use of felt belts for drying is the rewet of the cellulosic structure as the belt and structure leave the nip point of the press rolls. When the pressure of the rolls is removed, water present in the felts can move back into the cellulosic structure.
Generally, a felt belt is not preferred for the production and drying of a cellulosic fibrous structure having multiple regions. The uniform compression of the fibrous structure by the felt belts reduces the density differences between the regions. Other drying means, which avoid this overall compression of the cellulosic fibrous structure, are more preferable.
Drying cellulosic fibrous structures through vacuum dewatering, without the aid of felt belts, is known in the art. Vacuum dewatering of the cellulosic fibrous structure mechanically removes moisture from the cellulosic fibrous structure while the moisture is in the liquid form. Furthermore, the vacuum deflects discrete regions of the cellulosic fibrous structure into the structure of the drying belts. Such deflection strongly contributes to having different amounts of moisture in the various regions of the cellulosic fibrous structure. Similarly, drying a cellulosic fibrous structure through a vacuum-assisted capillary flow, using a porous cylinder having preferential pore sizes is known in the art as well. Examples of such vacuum-driven drying techniques are illustrated in commonly assigned U.S. Pat. No. 4,556,450 issued Dec. 3, 1985 to Chuang et al. and U.S. Pat. No. 4,973,385 issued Nov. 27, 1990 to Jean et al.
In another drying process; considerable success has been achieved drying the web of cellulosic fibrous structures by through-air drying. In a typical through-air drying process, a foraminous air-permeable belt supports the web to be dried. Hot air flows through the web, then through the air-permeable belt or vice versa. The air flow principally dries the web by evaporation. Regions coincident with and deflected into the foramina in the air-permeable belt are preferentially dried and the caliper of the resulting cellulosic fibrous structure is increased. Regions coincident the knuckles in the air-permeable belt are dried to a lesser extent.
Several improvements to the air-permeable belts used in through-air drying have been accomplished in the art. For example, the air-permeable belt may be made with a high open area (at least twenty-five percent). Or, the belt may be made to have reduced air permeability. Reduced air permeability may be accomplished by applying a resinous mixture to obturate the interstices between woven yarns in the belt. The drying belt may be impregnated with metallic particles to increase its thermal conductivity and reduce its emissivity or, alternatively, the drying belt may be constructed from a photosensitive resin comprising a continuous network. The drying belt may be specially adapted for high temperature air flows, of up to about 300 degrees C. (575 degrees F.). Examples of such through-air drying technology are found in U.S. Pat. No. Re. 28459 reissued Jul. 1, 1975 to Cole et al., U.S. Pat. Nos. 4,172,910 issued Oct. 30, 1979 to Rotar, 4,251,928 issued Feb. 24, 1981 to Rotar et al., commonly assigned U.S. Pat. No. 4,528,239 issued Jul. 9, 1985 to Trokhan, and U.S. Pat. No. 4,921,750 issued May 1, 1990 to Todd.
Additionally, several attempts have been made in the art to regulate the drying profile of the cellulosic fibrous structure while it is still a web to be dried. Such attempts may use either the drying belt, or an infrared dryer in combination with a Yankee hood. Examples of profiled drying are illustrated in U.S. Pat. Nos. 4,583,302 issued Apr. 22, 1986 to Smith and 4,942,675 issued Jul. 24, 1990 to Sundovist.
The foregoing art does not address the problems encountered when drying a multi-region cellulosic fibrous structure. For example, a first region of the cellulosic fibrous structure, having a lesser absolute moisture, density or basis weight than a second region, will typically have relatively greater air flow therethrough than the second region. This relatively greater air flow occurs because the first region of lesser absolute moisture, density or basis weight presents a proportionately lesser flow resistance to the air passing through such region. The greater air flow results in the preferential drying of these regions. Thus, vacuum drying and through-air drying each result in a web with a problematic non-uniform moisture distribution.
The ideal moisture distribution for a multiple region fibrous web is one where the different regions of the web simultaneously reach a uniform moisture level at the completion of the drying process.
As an example of the problem of achieving such a simultaneous uniform moisture distribution, when typical multi-region paper webs are transferred to a Yankee dryer, the webs have a non-uniform moisture distribution. The regions with a higher moisture content can be those in contact with the Yankee dryer. The Yankee dryer and hood combination preferentially dries those regions in contact with the dryer. The regions not in contact with the Yankee dryer, with much lower moisture content, are more completely dried by the Yankee hood. The ideal moisture distribution should be such that the moisture level of regions not in contact with the Yankee is somewhat less than those in contact with the dryer so there is uniform moisture at the end of the drying process. It is desirable to achieve such a moisture distribution without reducing the throughput speed of the Yankee and the entire process.
It would be advantageous to be able to adjust the differences in the moisture content between the regions in direct contact and those not in direct contact with the Yankee, prior to the Yankee, or other drying means in order to optimize the performance of the drying system, and to achieve higher throughput speeds.
Another drawback to the approaches in the prior art (except those that use mechanical compression, such as felt belts) is that each relies upon supporting the cellulosic fibrous structure to be dried. Air flow is directed towards the cellulosic fibrous structure and is transferred through the supporting belt, or, alternatively, flows through the drying belt to the cellulosic fibrous structure. Differences in flow resistance through the belt or through the cellulosic fibrous structure, amplify differences in moisture distribution within the cellulosic fibrous structure, and/or create differences in moisture distribution where none previously existed.
One improvement in the art which addresses this problem is illustrated by commonly assigned U.S. Pat. No. 5,274,930 issued Jan. 4, 1994 to Ensign et al. and discloses limiting-orifice drying of cellulosic fibrous structures in conjunction with through-air drying, which patent is incorporated herein by reference. This patent teaches an apparatus utilizing a micropore drying medium which has a greater flow resistance than some portion of the interstices between the fibers of the cellulosic fibrous structure. The micropore medium is therefore the limiting orifice in the through-air drying process so that at least a more uniform moisture distribution is achieved in the drying process.
Yet other improvements in the art which address the drying problems are illustrated by commonly assigned U.S. Pat. Nos. 5,543,107 issued Aug. 1, 1995 to Ensign et al.; 5,584,126 issued Dec. 19, 1996 to Ensign et al.; and 5,584,128 issued Dec. 17, 1996 to Ensign et al., the disclosures of which patents are incorporated herein by reference. The Ensign et al. ""126 and Ensign et al. ""128 patents teach multiple zone limiting orifice apparatuses for through air drying cellulosic fibrous structures. However, Ensign et al. ""126, Ensign et al. ""128, and Ensign et al. ""930 do not teach how to increase the amount of interstitial water in the web that is brought into hydraulic contact with the pores of the limiting orifice medium.
Applicants have unexpectedly found that pressing the wet web against the limiting orifice medium, while drawing a vacuum through the medium greater than the breakthrough pressure of the pores, promotes greater and more rapid and more complete dewatering of the web.
Accordingly it is an objective of this invention to provide a method of dewatering a fibrous web that results in greater dewatering and a more uniform moisture distribution. It is a further objective of this invention to provide a method of dewatering that results in a reduction of the residence time necessary to dewater the web, and reduces the rewetting of the web at the exit of the pressing nip.
A method comprising supporting a web on a fluid permeable carrier; pressing the carrier and the web between a pressing device and a limiting orifice medium, and drawing a vacuum greater than the breakthrough pressure of the pores of the limiting orifice medium through the medium, the web and the carrier. Such a medium may be comprised of a plurality of pores with web contacting and non-web contacting surfaces.
In one embodiment, the pressing device may also be a fluid permeable roller. A positive pressure may be applied through the roller to the carrier and the web. Alternatively, suction may be applied to the carrier and the web through the pressing roller while suction is also applied through the limiting orifice medium. It is also possible to employ a fluid permeable pressing roller where no pressure differential is utilized across the pressing roller. It is further possible that such a fluid permeable pressing device may also be a limiting orifice medium.
The outer surface of the press roller may be soft enough that the surface will deform in the nip increasing the residence time of the web and carrier in the nip point. Overall residence time may also be increased through the use of multiple nip points.
The pressing device, the carrier and the limiting orifice medium may be heated either individually or in combination to improve the drying performance of the method.